Jet aircraft and modern naval vessels are commonly powered by turbojet engines. Such engines operate on the principal of accelerating a small mass of gas to a high velocity, thereby producing a reaction required to push the craft forward. The basic requirements of producing the motive reaction include compressing an air charge, burning fuel within the compressed air, and exhausting the hot gases through a nozzle. The basic components of a turbojet engine include an air inlet (or diffuser), compressor stages where the inlet gases are compressed, a combustor where the fuel is burned, a turbine for providing rotative power to the compressor, and the outlet nozzle. The exiting of the hot gases from the combustor through the turbine provides the turning force to the turbine for turning the compressor stages.
Axial-flow compressors are used on nearly all turbojet, turbofan, and turboprop engines for aircraft. Axial flow compressors are made up of several "stages", with each stage being made up of a row of rotating blades (rotor) and a row of stationary blades (stator). The blades are detachably carried by blade carrying hubs, and can be individually removed upon disassembly of the engine for inspection. The stator blades decelerate the air, increase the air static pressure, and direct the air at the proper angle into the succeeding row of rotor blades. As the air passes from each stage, the pressure and temperature of the air are increased. The heat of combustion is added in the burner section of the engine.
The turbine wheel, similar to the compressor stages, is comprised of a number of individual blades that are keyed or slot mounted to a turbine hub. The turbine blades are exposed to extreme temperatures of between 1600.degree. and 2400.degree. F. It will be appreciated that both the turbine blades and compressor blades are subjected to extremely high stresses due to centrifugal and axial loads.
The efficiencies at which the turbine and compressor operate are very important to the overall thrust output of the engine. In particular, approximately two-thirds of the energy available in the hot gases produced in the combustor is absorbed by the turbine to drive the compressor. The material integrity of the compressor blades and turbine blades is accordingly critical to turbojet engine performance. As the blades deteriorate due to thermal wear, centrifugal stresses, and ingestion by the engine of foreign objects, less air is input and compressed in the engine, causing the engine to drop in fuel efficiency and to lose power. In the extreme, blade wear can cause engine failure.
The Federal Aviation Administration (FAA) has established requirements for the inspection of turbojet engine blades. The requirements mandate the inspection of the blades after a specified number of hours of operation of the engine. The requirements also set out the criteria by which a blade can be determined to be serviceable or nonserviceable. Conventional inspection methods include the use of plastic templates that fit over the blade for comparison of the blade to dimensions indicated on the template. In addition, mechanical calipers are used to measure various dimensions on the blade. Contact or mechanical gauging techniques, such as the use of calipers and templates, however, are susceptible to wear of the gauging device and resultant loss of accuracy over time. Moreover, such techniques require operator skill, are time intensive, are dependent on visual inspection, and are incapable of providing a direct input to a recording system.
Another problem that has been identified in conjunction with the contact or mechanical inspection of turbojet engine blades is the excessively high reject rate of serviceable blades. As many as 30-35% of blades initially rejected by the use of visual, contact mechanical gauging procedures have been found to be falsely rejected. Individual blades can cost from between $1,600 to $10,000 per blade, depending on the size of the engine the blade pertains to. With as many as 40 to 50 blades per stage, the cost of a high blade reject rate can quickly become prohibitive.